Cementitious settable compositions are used in the treatment and drilling of well bores extending from the surface of the earth to subterranean formations. Generally, the drilling of a well for the recovery of oil, gas or water from a subterranean formation involves placing and securing a casing in the well bore by cementing. In this operation, cement is pumped downward from the surface of the earth to the bottom of the well. After the cement (i.e. the cementitious settable composition) has been pumped between the casing and the well, pumping is discontinued and the cement is permitted to set in place.
In cementing procedures in a well bore, it is sometimes desirable that the cement slurry has a low density. Aside from being costly due to a high ratio of cement to water, high density cement slurries require higher pumping pressures in order to place them at the desired positions in the well'bore. Further, high density cement slurries impose high static and dynamic pressures upon not only the formation to be treated but upon the other formations as well. At the locations where these other formations are porous, the high pressures exerted in the well bore can result in loss of the cement slurry into the formations or loss of the water from the cement slurry into the formations. As a result, the cement can set in an unintended position in the well bore. Thus, when designing cementitious settable compositions for subterranean applications, design engineers should ensure that the compositions provide adequate compressive strength and zonal isolation. Aside from providing strength and zonal isolation one should ensure that the selected compositions meet all regulatory guidelines for zonal isolation for the respective area. Another area of concern is to ensure that the cementitious composition selected to provide zonal isolation has the capability to be seen by down-hole interpretation methods (bond logs). Yet another area of concern from a manufacturing standpoint is to ensure that the cementitious composition remains uniform and that materials that are used as fillers do not impact the intended development of mechanical strengths and the rheological properties of the cementitious composition.
In order to obtain cement slurries having adequate mechanical and rheological properties and a density equal to or less than about 13 pounds per gallon, it is known in the art to use lightweight additives such as, for example, microspheres (e.g. glass beads) and foaming agents, as a partial substitute for water. However, the use of lightweight additives significantly adds undesirable costs as well as industrial challenges to the preparation of slurries. Lightweight additives are generally the most expensive materials in low density cementitious compositions. Further, the use of foaming agents or other similar chemicals for reducing the density of cement slurries may create hazards and pose environmental concerns.